1. Field of the Invention
The present invention relates to a pressure rotating member which is used in a heating device such as a thermal fixing apparatus that clamps a material to be heated therebetween and conveys the material therethrough to heat the material; a method for manufacturing the same; and a heating device using the same.
2. Description of the Related Art
In an electrographic apparatus, a heating device is used which includes a heating member and a pressurizing member that is arranged so as to face the heating member, as a heating device for fixing an unfixed toner image which has been formed on a recording material, on the recording material.
When such a heating device is made to adapt to the recording materials having various sizes, it becomes a problem that temperature rises in a region of the heating member, with which a recording material having a small size (for instance, A-4 size paper) does not come in contact. A specific example of such a region includes an end region in a cross direction of the heating member. This problem is occasionally referred to as “non-recording material-contacting area's temperature rise.”
Specifically, when a recording material having a relatively smaller width than the width of the heating member of a fixing apparatus continuously passes through a nipping portion formed of the heating member and the pressurizing member, a temperature in the region rises with which the recording material in the nip does not come in contact. This phenomenon occurs because the heat transferred from the heating member is not absorbed by the recording material and/or toner on the recording material, in the region with which the recording material in the nip does not come in contact.
Such a phenomenon occasionally causes the deterioration and/or deformation of the pressurizing member and the heating member. In addition, when paper having a large size passes through the nip which is in such a state that the temperature in the region has excessively risen with which paper having a small size does not come in contact, the toner on the paper having the large size is excessively melted, and offset occasionally occurs.
Such a problem more easily occurs as the speed (process speed) of the image output of a printer increases. Specifically, a period of time during which the recording material passes through the nip becomes shorter as the speed of the image output becomes higher, and accordingly the heating member needs to transfer sufficient heat to the toner image in a shorter period of time. The above problem occurs because the temperature of a fixing roller needs to be set at higher temperature for the sufficient heat transfer.
On the other hand, in an electrophotographic image-forming apparatus, it is desired to shorten a period of time necessary for the output of a first image after startup (hereinafter referred to as “first printout time”), and further shorten a period of time for raising the temperature of the nipping portion of the heating device to a temperature necessary for fixing the toner (hereinafter also referred to as “warm-up time”) in order to reduce power consumption.
For this reason, it is performed to reduce thermal conduction by making the elastic layer of the pressurizing member contain a void. Specifically, a heat quantity to be transferred to the pressurizing member from the heating member when the operation of the heating device is started is controlled to be small, and a speed of the temperature rise of the heating member is enhanced by the reduction of the thermal conduction of the pressurizing member.
Here, the following three methods are known as methods for forming the elastic layer having the void therein.
In Japanese Patent Application Laid-Open No. 2008-150552, a foaming agent is mixed into uncrosslinked silicone rubber to cause foaming and cure in the silicone rubber, and thereby the void is formed therein. In Japanese Patent Application Laid-Open No. 2001-265147, a hollow filler is previously mixed into uncrosslinked silicone rubber to thereby form the void after the uncrosslinked silicone rubber has been molded and cross-linked. In addition, in Japanese Patent Application Laid-Open No. 2002-114860, a water-absorbing polymer in which water is absorbed is dispersed in uncrosslinked silicone rubber, and the void is formed by a dehydration reaction occurring when the uncrosslinked silicone rubber is cross-linked. However, the reduction of the thermal conduction of the pressurizing member results in further causing the acceleration of the temperature rise in a noncontact region of the recording material having the small size in the previously described nip.
Accordingly, it has been difficult to balance the reduction of the non-recording material-contacting area's temperature rise in the nip with the shortening of the warm-up time of the nip.
Incidentally, in Japanese Patent Application Laid-Open No. 2002-351243, a high thermally-conductive rubber composite is used in which a fibrous filler is blended into the elastic layer of the pressure rotating member to enhance the thermal conduction in the rotational axis direction of the member, and it is attempted to thereby suppress the non-recording material-contacting area's temperature rise. In addition, it is described that a porous elastic layer is provided in the lower layer of the elastic layer to lower the thermal conduction in the thickness direction of the elastic layer, and the shortening of the warm-up time can also be expected.